Tetraalkyllead recovery from an anhydrous reaction mass



United States Patent 3,384,559 TETRAALKYLLEAD RECOVERY FROM AN ANHYDROUS REACTION MASS Frank M. Hopkins, Baton Rouge, La., assignor to Ethyl Corporation, New York, N.Y., a corporation of Virginia N0 Drawing. Filed Aug. 4, 1964, Ser. No. 387,487 Claims. (Cl. 203-49) ABSTRACT OF THE DISCLOSURE This invention relates to the manufacture and recovery of tetraalkyllead compounds. More particularly, the invention relates to a new and novel process and technique whereby tetraalkyllead compounds are recovered from reaction masses resulting from the reactionof an alkali metal lead alloy and an alkyl halide by stripping at least the tetraalkyllead product from a reaction mass with an inert gas and recovering the product from inert gas.

The tetraalkyllead compounds, particularly tetraethyllead and tetramethyllead, are well known organometallics highly effective as antiknock additives for hydrocarbon fuels for internal combustion engines. These valuable chemicals are synthesized, normally, by the reaction of an alloy of lead and an alkali metal, especially sodium, with an alkyl halide, the alkyl group thereof being that which is desired for attachment in the tetraalkyllead compound. Such a reaction, illustratively, for the synthesis of tetraethyllead, is represented by equation:

4NaPb 4 0211501 (C2H5)4Pl3 4NaCl 3 Yb Monosodiun ethyl tetraethyl sodium lead lead alloy chloride lead chloride Yields of the order of 80 to 90 percent of the foregoing equation are usually achieved.

It Will be clear from the foregoing equation, that the products of a synthesis reaction include not only the desired tetraalkyllea-d but appreciable quantities of unreacted or excess lead, as well as the alkali metal halide resultant from the reaction. In addition to these components, minor quantities of the alkyl halides, and the lead alloy fed for the synthesis are usually present in the reaction mass.

The substantial quantity of lead in the products of the reaction must be recovered. It is present as very finely subdivided particles of high surface characteristics, resultant from the reacting-out of the alkali metal, such as sodium, from the alloy in which it is originally charged.

Heretofore, it has been generally considered that, to recover the alkyllead products from a reaction mixture, a steam distillation of the tetraalkyllead was the preferred practice. The steam distillation has been conducted in the presence of a substantial additional quantity of water.

Steam distillation has been successfully used in thousands, and thousands of batch operations. While quite effective, certain problems have arisen which detract from its advantages. These are the fact that the steam distillation appears to be at a relatively inefficient basis, during a large proportion of the steam distillation cycle. For example, the rate at which tetraethyllead is distilled decreases greatly at the end of the usual batch operation. An inordinately large portion of a distillation cycle is therefore required for a minor part of the recoverable tetraethyllead. These results cannot be explained, because with a theoretically ideal steam distillation the rate of recovery would be uniform throughout the cycle. There is considerable evidence that a steam distillation can never approach ideal operation. The dropping off of the distillation rate is thought to result from entrapment of liquid ice tetraethyllead within agglomerated particles which are predominantly lead metal. Accordingly, regardless of the operability of the steam distillation recovery process, a limitation to its possibilities appears to exist. Additionally, the presence of an aqueous phase tends to cause agglomeration of the lead particles so that the addition of anti-agglomerates to the steam distillation operation has been necessary, which represents an additional operating and material expense. Moreover, the presence of the antiagglomerates causes the formation of undesired by-products. Illustrative of these prior art techniques are Patents 2,004,160 by Downing et a1., 2,513,654, by Krohn, and Patent 2,513,659 by Madden. The formation of these agglomerates is also accompanied by numerous other drawbacks which are known in the art. A further disadvantage of the presence of the aqueous component is that there is a tendency for the alkali metal content of the reaction mass to be converted to the corresponding alkali hydroxide.

Despite the fact that the problems associated with steam distillation have to a large extent been solved, or minimized, in many commercial installations the steam distillation is an operative limitation to the efiiciency of the process.

The general object of the present invention is to provide a new and novel process for the recovery of tetraalkyllead such as tetraethyl'lead, tetramethyllead, tetrapropyllead, or mixed lead alkyls when generated by a direct synthesis employing a plurality of alkyl halide alkylating agents. More particularly, a preferred object of the present invention is to provide an improved recovery process which is devoid of any aqueous liquid phase and which provides an efficient and rapid recovery of the tetraalkyllead. Other objects will appear hereinafter.

In the most general form, the present invention consists essentially of contacting a reaction mass of the character described above with an essentially inert contacting gas or vapor under pressure and velocity conditions Whereby the tetrahydrocarbon lead compound is stripped or vaporized into the inert -gas and whereby a separate aqueous liquid phase is avoided during the vaporization process. Various manipulative techniques are available for effecting the above described contacting, which contacting is continued, with respect to the solids present, until a sufficiently high degree of recovery of the tetraalkyllead component is achieved. In certain cases, particularly, in the case of a tetramethyllead synthesis reaction mass, the process concurrently recovers organic normally liquid diluents provided in the synthesis reaction as will be illustrated hereinafter.

Various manipulative techniques are available for implementing the invention defined above. In a preferred feature of the invention the solid components of the reaction mass are fluidized at least in part by the inert gas. In one mode of operation, the reaction mass is charged by batch operation to a vertical chamber, and the inert, preheated gas is fed below this charge at a sufficient velocity to accomplished fiuidization of the solids present. In a variation of this fluidized technique, the contacting chamber is a vertically elongated drum and the solids are vertically transported by the gas velocity. An overflow arrangement is provided, whereby the solids, substantially denuded of the tetraalkyllead components, are discharged from the treated phase. In another class of embodiments, an elongated relatively small diameter contacting tube is provided and the solids are swept along through the tube or bank of tubes at a relatively high velocity by the contacting gas.

Another method of operation comprises feeding the contacting gas through a bed of the reaction mass at a gas velocity which is insuflicient to suspend the reaction mass particles. The reaction mass particles may be either stationary or may be agitated by some mechanical means such as a stirrer or vibrator. Other methods for contacting the gas with the reaction mass will be apparent to one skilled in the art.

Following the above described class of treatments, the effluent hot gas and the accompanying vaporized tetraalkyllead may then be passed to a cooler arrangement for Iractional condensation of the tetraalkyllead components or for its recovery in an equivalent manner.

Various essentially inert contacting gases can be employed for the operation. Excellent results are obtained with nitrogen. More expensive rare gases can be used with full success but are generally not desired, even though provision is normally made for the recovery and recirculation of these gases. Although not generally preferred, air may be employed.

A particularly preferred class of embodiments uses a vaporized alkyl chloride corresponding to the alkylating agent previously used in the synthesis reaction as such. Thus ethyl chloride vapor, methyl chloride vapor or mixtures of said vapors are used veryetfectively in this class of embodiments.

One surprising feature of the present invention is the fact that a more rapid and efiicient recovery is generally experienced solely by the gas-solid contacting. This is surprising in view of the fact that the steam distillation technique of the prior art would be expected to provide the most efiective heat transfer because the reaction mass in steam distillation is immersed directly in boiling water. Consequently, the most rapid distillation of the desired alkyllead component would have been expected by the steam distillation technique.

To illustrate the benefits of the invention and the techniques of operation, thereof the following examples are given.

EXAMPLES I TO V In the following series of operations, the reaction mass was produced by reacting a comminuted monosodium lead alloy, NaPb, with a substantial excess of ethyl chloride. Upon completion of the reaction, the excess ethyl chloride was vented, leaving a reaction mass having the following approximate composition.

Component: Wt. percent Tetraethyllead 24 Sodium chloride 22 Lead 54 Also includes minor ingredients such as unreacted alloy and ethyl chloride.

The lead was in the form of fine granules having an average particle size of less than 500 microns.

Portions of this reaction mass were inserted in a vertical tube, having an inlet at the bottom for pre-heated gas. A gas line overhead from this unit passed to a watercooled condenser, and any condensate therefrom was collected in a product receiver.

In a series of operations, a stream of pre-heated nitrogen Was passed into the treating chamber at a velocity at least suflicient to fiuidize the solids. The results obtained are tabulated below:

Nitrogen Example Temperature, Velocity, Utilized,* Percent TEL C. ft./sec. 1b. [1b. to Recovery 90% Recov.

102 0. 310 I. 95 98 101 0. 174 1. 9G 96 102 0. 298 2. 35 97 86 0. 298 4. 9 97 102 0. 234 l. 97 95 *The values of nitrogen utilization were determined when 90 percent of the tetraethyllead was recovered, and are reported as pounds of nitrogen used per pound of tetrnethyllead recovered. In the foregoing Ex- The tetraethyllead was recovered from a reaction mass utilizing ethyl chloride as the contacting gas. The reaction mass was produced in conventional manner by reacting about .227 lb. mole of ethyl chloride with about .137 lb. mole of a monosodium lead alloy in a reactor. After the reaction had been completed, unreacted ethyl chloride was vented. Next 18.5 lbs. of the vented reaction mass having the approximate composition,

Component: Wt. percent Tetraethyllead 23 Sodium chloride 17.2 Lead 59.8

was transferred to vaporizing or stripping apparatus by means of a screw feeder. The r.p.m. of the screw feeder was set to deliver about 400 grams per minute of the reaction mass. The vaporizer comprised 5 double pipe heat exchangers having 4 inch internal tubes, schedule 40 thickness steel. The heat exchangers were arranged in a trombone arrangement. The total length of /4 inch pipe was about 25 feet. The product of the vaporizer was transferred to a separator for separating the solids and gases. superheated ethyl chloride was supplied into the system by feeding liquid ethyl chloride to a steam heat exchanger. The superheated ethyl chloride was at a temperature of about 110 C. as it entered the vaporizer. The ethyl chloride was fed concurrently with the reaction mass. The ethyl chloride was fed at a rate of 100 lbs. per hour. All points throughout the vapor stripper were maintained at a temperature of 103 C. to 112 C. by control of the jacket temperature. The reaction mass was conveyed through the vaporizer by the velocity of the ethyl chloride being passed through the vaporizer. By this process over 93% by weight of the tetraethyllead was removed from the reaction mass to the ethyl chloride vapor.

EXAMPLE VII In another run a hydrocarbon gas, ethane, is substituted for the nitrogen of Example I. The ethane is fed to the treating chamber at a temperature of about 110 C. and at a velocity of about 0.4 ft./sec. A high percent recovery of the tetraethyllead is obtained.

EXAMPLE VIII In the next run methyl chloride is reacted with NaPb in a reactor containing toluene to produce a reaction mass. The methyl chloride is vented leaving a reaction mass containing the approximate composition.

Component: Wt. percent Tetramethyllead l8 Toluene 4.5 Sodium chloride 17.0 Lead 60.5

The reaction mass is fed to the vaporizing apparatus described in Example VI. Methyl chloride gas at a temperature of about C. is passed through the stirred mass with the gas temperature being approximately 75 C. and the temperature of the reaction mass being approximately 70 C. The tetramethyllead is vaporized into the methyl chloride gas and thereafter separated therefrom.

EXAMPLE IX Example VIII is repeated with the exception that only 0 one half of the methyl chloride is vented prior to vaporizing with methyl chloride. The remaining methyl chloride and the tetramethyllead are vaporized and recovered.

EXAMPLE X Example VIII is repeated with a reaction mass resulting from the reaction of NaPb With an equimolar mixture of methyl chloride and ethyl chloride. The reaction mass contacted with benzene gas at a temperature of about C. A high yield of a distribution of methyl and ethyllead compounds is recovered from the benzene gas.

EXAMPLE XI The reaction mass of Example VIII, prior to venting, is placed in a treating chamber having an agitator to stir the reaction mass. Methane is used as the contacting gas. The tetramethyllead, toluene and the methyl chloride are stripped into the methane gas. The tetramethyllead is separated from the methyl chloride and toluene, and the methyl chloride is returned as the contacting gas.

EXAMPLE XII Example V1 is repeated with the exception that the reaction mass is not vented prior to feeding to the vaporizer. The contacting gas, ethyl chloride, vaporized not only the tetraethyllead but also the ethyl chloride from the reaction mass.

As may be seen from the above examples, high recoveries of tetraalkyllead compounds are achieved.

It will be understood that the reaction mass solids to be treated according to this invention are not discrete, smooth particles of lead, but high surface, adsorptive particles, the apertures and interstices having tetraalkyllead and alkali halide entrapped therein. In appearance, the reaction mass may appear as a dry, granular or sandy material.

However, it is not necessary that the reaction of the lead alloy and alkyl chloride be carried out to the usual commercial end point. For example, in some instances there may be substantially more unreacted lead alloy or alkyl halide present whereby the reaction mass may be fairly fluid in appearance at the time it is contacted with the contacting gas. Therefore, the alkyl halide may be stripped or vaporized from the reaction mass with the contacting gas at the same time the tetraalkyllead is vaporized, Typically the reaction mass to be treated will contain, exclusive of any alkyl halide or inert diluent, in percent by weight from about 14 to 26 percent tetraalkyllead, from about 51 to 65 percent lead and about from 14 to 25 percent alkali metal salt. The preferred composition will contain by weight from about 17 to 25 percent tetraalkyllead, from about 53 to 61 percent lead and from about 16 to 23 percent alkali metal salt. Additionally, the reaction masses to be treated may contain generally up to about 50 percent by weight alkyl halide, based on the total weight of the reaction mass including the alkyl halide. As discussed below, the reaction mass may also contain diluents such as toluene. After removal of, the tetraalkyllead, the treated mass will generally contain less than 10 weight percent tetraalkyllead, and from about 16 to 30 weight percent alkali metal salt.

A preferred feature of this invention is the use of the process of the invention in the recovery of alkyllead compounds from reaction masses which reactions have been conducted in the presence of an added hydrocarbon. EX- amples of such reaction masses are disclosed in US. 3,049,558 issued Aug. 14, 1962; which patent is herein incorporated by reference. For example, according to US. 3,049,558 tetramethyllead is produced by the reac tion of a sodium lead alloy with methyl chloride in the presence of a hydrocarbon liquid which is inert to the reacting materials at the conditions of operation. According to this patent, the added hydrocarbons should have a boiling point of from about 90 to 150 0, preferably within the range of 110 to 150 C. Preferred hydrocarbons are such as toluene and the preferred proportions of the hydrocarbon are from about to about 25 parts by weight of hydrocarbon per 100 parts of lead in the alloy. In this preferred feature of the invention wherein reaction masses containing hydrocarbons are treated according to the present invention, the present invention is not restricted to he use of reacion masses disclosed in U.S. 3,049,558. For instance, the tetraalkyllead compounds may be tetramethyllead, mixtures of tetramethyllead and tetraethyllead, or redistributed mixtures of tetramethyllead and tetraethyllead. Other modifications of the process disclosed in US. 3,049,558 may also be made. Also hydrocarbon diluents boiling at temperatures of less than 90 C., such as 25 C.

or higher, may be employed as the hydrocarbon diluent during the reaction to form the tetraalkyllead. As in the case of the hydrocarbons boiling at 90 C. or greater, the lower boiling hydrocarbons may be allowed to remain with the tetraalkyllead during vapor stripping; that is, to the extent that the lower boiling hydrocarbons will remain wih the tetraalkyllead. Typical hydrocarbon diluents could be aliphatic such as pentane, aromatic such as benzene or cycloaliphatic such as cyclohexane, and mixtures thereof or mixed types. Moreover, it is not necessary that the added diluent be a hydrocarbon so long as the diluent is essentially inert under the conditions of reaction.

The process of vaporization of the tetraalkyllead compound may be conducted at atmospheric, superatmospheric or subatmospheric pressure. For practical reasons the process is generally conducted at about atmospheric pressure. However, improved results may be obtained by operating under a vacuum. Generally, the pressure will be from 20 inches vacuum to about 300 p.s.i.g. Excellent results may be obtained at pressures between about atmospheric and 25 or 50 p.s.i.g.

The contacting gas will be at a temperature high enough to vaporize generally at least about 90 weight percent of the tetraalkyllead from the reaction mass. Suitable temperatures are from 25 C. to 200 C., but preferably the gas will be at a temperature of about to 150 C.

The preferred contacting gases will be essentially anhydrous, and the reaction mass to be treated will also preferably be essentially anhydrous. The contacting gas generally will be normally gaseous under the conditions of contacting with the reaction mass and should be essentially inert. The contacting gases will suitably have a boiling point at atmospheric pressure of less than 175 C., and ordinarily less than 125 C. Preferably, the contacting gas will have a boiling point at atmospheric pressure of less than C. The higher boiling point stripping gases are especially suitable when the gas contacting vaporization process is operated under a vacuum. By essentially inert gas is meant that the contacting gas should not have an appreciable rate of reaction with the alkali metal salt or the tetralkyllead in the reaction mass under the conditions of reaction. Also the gas should not cause significant agglomeration of the particles of the reaction mass. Furthermore, the gas should be one that does not dissolve out a significant quantity of the sodium chloride from the reaction mass during the process of vaporization of the tetraalkyllead. Ordinarily at least weight percent of the alkali metal salt originally present in the reaction mass being treated just prior to contacting with the gas will be present after treatment with the contacing gas.

The velocity of the contacting gas will generally be from about .05 ft./sec. to sonic velocity, but ordinarily will be from 0.10 to 500 or 600 ft./sec. However, the velocity will depend on the type of process utilized, e.g. fluid bed.

The quantity of the contacting gas utilized may be varied considerably depending upon such factors as the temperature of the reaction mass, the temperature of the contacting gas, the partial pressure of the tetraalkyllead compound, the total pressure on the system, the number of contacting stages, the velocity of the reaction mass and the contacting gas, and the like. A preferred embodiment is to utilize only sufiicient contacting gas in order that the contacting gas approaches saturation with the tetraalkyllead compound. Typical ratios of contacting gas used to tetraalkyllead recovered are from about 3 to 25 pounds of contacting gas per pound of tetraalkyllead recovered.

The preferred alkyl halides which may be employed as the contacting gas will have from 1 to 5 carbon atoms and may contain one or more halogen atoms. Examples of suitable alkyl halides are methyl chloride, ethyl chloride, ethyl bromide, ethylene dichloride, vinyl chloride, n-propyl chloride, ethylene chlorobromide, sym-tetra chloroethane, mixtures thereof and the like. Particularly preferred are methyl chloride, ethyl chloride and mixtures thereof.

Hydrocarbons may be utilized as the contacting gases. Suitable hydrocarbons are aliphatic and aromatic hydrocarbons such as those having from 1 to 10 carbon atoms, but preferably having from 1 to 7 carbon atoms such as methane, ethane, propene, n-butane, isopentane, isooctane, benzene, toluene and the like.

By the process of this invention generally at least 80 weight percent of the tetraalkyllead present in the reaction mass is vaporized into the gas being contacted with the reaction mass and preferably at least 90 weight percent of the tetraalkyllead is vaporized into the gas.

A particular and surprising result of the present invention is the higher degree of theoretical efficiency attained than is realized by a steam distilation process. In the steam distillation process higher amounts of steam are required to recover the tetraalkyllead than would be theoretically expected. In contrast, in the present invention the overall efiiciency is high. Another unanticipated result of the present procedure is in fact that the distillation rate is maintained at a high level, approaching the theoretical, for a longer portion of the distillation cycle than is the case with a steam distillation. In the case of a steam distillation, for example, the rate of the alkylle-ad vaporization drops off rapidly after about 80 percent of the tetraethyllead has been distilled. In contrast in operations using an inert gas according to the present invention, a high rate of vaporization is maintained during a longer portion of the total recovery treatment. For example, whereas, in a steam distillation operation, the falling rate period is prolonged, in a typical recovery operation according to the present process, the falling rate period may be quite short.

Numerous modifications of the process of the invention will be evident to one skilled in the art. For example, instead of a single operation for contacting the gas with the reaction mass this may be done in two or more steps. Moreover, additional heat may be added to the reaction mass or to the stripping gas prior to or during the recovery operation. Another modification of the process is that the alkylating agent of the reaction mass may be either partially or completely vented prior to the contacting with the contacting gas.

What is claimed is:

1. A process for the recovery of tetraalkyllead product from an essentially anhydrous reaction mass having the appearance of a substantially dry particulate material and obtained from the reaction of an alkali metal-lead alloy and an alkyl halide and said reaction mass comprising tetraalkyllead, subdivided lead particles and an alkali metal salt which comprises, in combination,

(a) stripping at least said tetraalkyIlead product from said essentially anhydrous reaction mass having the appearance of a substantially dry particulate material with an essentially anhydrous inert gas at a temperature of from about C. to about 200 C. to leave behind substantial portions of the subdivided lead particles and the alkali metal salt, and

(b) recovering at least said tetraalkyllead product from said essentially anhydrous inert gas.

2. The process of claim 1 further characterized by said essentiallly anhydrous inert gas being an alkyl halide having from one to about five carbon atoms.

3. The process of claim 1 further characterized by said essentially anhydrous reaction mass comprising an essentially anhydrous diluent and said essentially anhydrous diluent being stripped and recovered with said tetraalkyllead product.

4. The process of claim 3 further characterized by said essentially anhydrous diluent being a hydrocarbon having a boiling point of from about 25 C. to about C.

5. The process of claim 1 further characterized by said essentially anhydrous reaction mass comprising an essentially anhydrous alkyl halide and said essentially anhydrous alkyl halide being stripped and recovered with said tetraalkyllead product.

References Cited UNITED STATES PATENTS 1,975,171 10/1934 Parmelee 260-437 2,415,444 2/1947 Ruddies 260437 2,574,759 11/1951 Rodekohr et al 260437 2,661,361 12/1953 Grandjean 260437 2,723,227 11/1955 Rudy 260-437 3,270,042 8/1966 Collier 260-437 NORMAN YUDKOFF, Primary Examiner. D. EDWARDS, Assistant Examiner. 

